1. FIELD OF THE INVENTION
The invention relates to control of phased array systems generally and more particularly to means for generating multi-phased radio frequency signals for such arrays.
2. DESCRIPTION OF THE PRIOR ART
The prior art in respect to phased array antennas and the technique for generating the required multi-phase excitation signals in controllable fashion, are extensively described in the technical literature. The text "Phased Array Antennas" by Oliver and Knittel (proceedings of the 1970 Phased Array Antenna Symposium) provides a good prior art insight into the general design of phased arrays, the requirements for excitation, and the limitations encountered. That text was published by Artech House, Inc., Dedham, Mass., and is further identified by Library of Congress Catalog Card No. 73-189392.
In addition, the text "Radar Handbook" by Merrill I. Skolnik, (McGraw Hill 1970) also provides considerable insight and background information in respect to the design of phased array systems.
In general, a phased array, which provides maximum scanning flexibility and random, inertialess, beam-pointing capability, involves the individual excitation of the radiating elements of the arrays, or at least individual rows or columns of elements treated discretely in respect to the phase of the RF excitation thereof. In some of the most advanced and most flexible phased array systems, two-dimensional arrays, such as planar arrays, are used which require individual excitation of all or substantially all of the elements in order to provide a pencil-beam with pointing flexibilty desired throughout a solid angle of coverage.
What may be referred to as the classical approach to the problem involves the use of controllable individual radio frequency phase shifters between the source of transmittable RF, and each of the aforementioned array radiating elements (antenna elements). Chapter 12 of the aforementioned Radar Handbook reference describes known types of controllable phase shifters available for the purpose. These include the so-called ferrite phase shifters, and those employing semiconductor diodes. The former can provide either stepped or continuously variable phase shift within recognized limits in response to a digital or analog type control signal, whereas the latter generally provide phase shift in discrete steps (usually digitally controlled). The manner of digital or analog control is explained in the text aforementioned.
"Random" beam pointing arrays have been constructed employing these techniques, however, the result has been very expensive apparatus of large size and considerable weight. Because of that fact, there has been considerable incentive for the development of simplifications to reduce the size and complexity of phased array control systems.
Not only have the prior art systems required the provision of large numbers of phase shifters (on a one-for-one basis to the array elements), but these devices and their driving circuitry have been relatively complex sub-systems of themselves.
For example, in the aforementioned Radar Handbook, Chapter 12, digital and analog latching phase shifter driver circuitry is shown. In addition to the complexity problem, relatively large amounts of electric power are required for the programmed operation of the prior art phased array scanning and beam pointing systems employing those approaches.
Still further, the prior art systems of the type very often do not provide phase placements for the individual elements of the array sufficiently accurate to provide uniform beam shape over a full range of beam pointing angles (scan angles).
The optical delay lines employed are preferably relatively inexpensive predetermined lengths of fiber optic cable, or single glass strands of that type. The art in respect to such light transmissive optical fibers is summarized and explained in an article entitled "Fiber Optic Communications: A Survey" by C. P. Sandbank, appearing in "Electrical Communication," Volume 50, Number 1, 1975, a technical journal published by International Telephone and Telegraph Corporation.
The manner in which the present invention deals with the disadvantages of prior art systems of the type to provide a novel and highly advantageous combination, which is relatively low in cost, size and weight, will be understood as this description proceeds.